It's possible that:
1) The host did not have their audio un-muted...
The host can unmute their audio by right-clicking on their name (in the user-list) and selecting Unmute.
2) Push-To-Talk (a bit like a walkie-talkie) is enabled...
By default, Push-To-Talk could be enabled for your Nefsis meeting.
Try pressing (and holding down) the "Push-To-Talk" button at the top of the user-list while speaking.
In either case, you should see the person's name in the user-list highlight green when their talking.
Hope this helps.